Bearing steel having improved fatigue durability and method of manufacturing the same

ABSTRACT

Disclose is an alloy composition for bearing steel having improved fatigue durability and a method of manufacturing the bearing steel comprising the same. The alloy composition comprises: based on a total weight of the alloy composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) constituting the balance of the weight of the alloy composition. Preferred alloy composition can provide improved strength, hardness, and fatigue life due to spheroidized carbide complex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2014-164102, filed on Nov. 24, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to an alloy composition for bearing steel that comprises a spheroidized complex carbide to improve hardness and fatigue durability, and a method of manufacturing the bearing steel comprising the alloy composition.

BACKGROUND

In vehicle industries, various environmentally-friendly vehicles have been developed with an object of reducing a discharge amount of carbon dioxide to about 95 g/km, that is a level of about 27% of a current amount thereof, until 2021 based on European regulations. Further, vehicle makers have developed a technology to reduce the vehicle size and improve fuel economy in order to satisfy about 54.5 mpg (about 23.2 km/1), which is a regulated value of corporate average fuel economy (CAFE) in the USA, until 2025.

Particularly, engines and transmissions of high performance and high efficiency technology have been developed for maximizing fuel economy of vehicles, and such technology development may include increased gears, a novel concept driveaway device, high efficiency of a two-pump system, a fusion hybrid technology, technologies which may be derived from an automatic/manual fusion transmission and a hybrid transmission, and the like.

Specialized steel used in the technology relating to the transmission has been used in a carrier, a gear, an annulus gear, shafts, a synchronizer hub, and the like of the transmission, and the specialized steel may be contained in a vehicle of about 58 to 62 wt % based on the total weight of the steel. Particularly, for a pinion shaft, a needle bearing, and an engine valve train-based roller swing arm of the transmission and the like, there has been a continuous demand for developing a high strength and high durable material due to the requirement of reducing weight and downsizing. For example, a SUJ2 steel containing about 1.5 wt % of chromium (Cr) has been used.

However, because of an increase in deterioration of parts due to downsizing and a size reduction of parts such as the bearing and the like, durability of the material may be reduced, which may further cause damage to a surface. Further, when there is no lubricating, a surface temperature may be increased and hardness in high temperature and high revolution environment may be reduced.

Particularly, for the bearing serving to fix a rotation shaft to a predetermined position, support a weight of the shaft and a load applied to the shaft, and rotate the shaft, a repeated load may be applied in proportion to a rotation number, and thus, in order to endure the repeated load, fatigue resistance, wear resistance, and the like of the material thereof may be required.

A bearing steel has been manufactured by, for example, steelmaking in a converter or an electric furnace, being refined in a ladle while a strong reducing atmosphere is maintained to reduce an amount of a non-metallic inclusion, and further being refined in a state where an oxygen content is reduced to about 12 ppm or less through a vacuum degassing process. Thereafter, the refined bearing steel is solidified into a cast slab or a steel ingot by a casting process, subjected to crack diffusion treatment in order to remove segregation and a large carbide existing at the center of the material, and rolled. Thereafter, an intensely slow cooling operation is performed in order to soften the material in a rolling factory to produce a bearing steel wire rod or rod material, and the produced wire rod is produced into bearing products through spheroidizing heat-treating, forging, quenching, tempering, and grinding processes and the like.

During the aforementioned manufacturing process, a heat-treating process for spheroidizing is mainly performed by diffusion at high temperatures, and formed globular particles are grown through a process that is similar to an Ostwald ripening principle to form a spheroidized tissue.

However, since the spheroidizing heat treating process requires growth of the globular particles, a long time is spent for spheroidizing, and thus a manufacturing cost is increased. As such, sufficient strength and durability life may not be obtained when deterioration of the bearing increases due to downsizing, a size reduction, and the like.

Therefore, the present inventor has tried to develop a bearing steel having improved physical properties such as strength and durability life by generating a spheroidized complex carbide, and a method of manufacturing the same.

SUMMARY OF THE INVENTION

In preferred aspects, the present invention provides a bearing steel having improved strength, durability life, and the like by forming a spheroidized complex carbide finely in the bearing steel. In particular, the bearing steel can be manufactured by adjusting a component and a content of an alloy of the bearing steel and controlling a process condition.

An exemplary embodiment of the present invention provides an alloy composition for a bearing steel that may comprise: based on a total weight of the alloy composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) being the balance of the weight of the alloy composition. The alloy composition may further comprise one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb).

Preferably, the alloy composition may further comprise an amount of about 0.006 wt % or less of nitrogen (N), an amount of about 0.001 wt % or less of oxygen (O), an amount of about 0.03 wt % or less of phosphorus (P), an amount of about 0.01 wt % or less of sulfur (S), and the like.

It would be understood that all the weight % (wt %) referred to herein is based on the total weight of the alloy composition, unless otherwise indicated.

Also provided is the alloy composition of the invention that may consist of, essentially consist of, or consist essentially of the components above. For example, the composition for bearing steel may consist of, essentially consist of, or consist essentially of: based on a total weight of the composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) being the balance of the weight of the alloy composition. Further, the alloy composition may consist of, essentially consist of, or consist essentially of: based on a total weight of the composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb), and iron (Fe) being the balance of the weight of the alloy composition.

In another aspect, the present invention provides a method of manufacturing a bearing steel. The method may comprise:

heat-treating a wire rod comprising an alloy composition at a temperature of about 720 to 850° C. for about 4 to 8 hours to spheroidize a complex carbide;

wire-drawing the heat-treated wire rod;

secondarily heat-treating the wire-drawn wire rod at a temperature of about 720 to 850° C. for about 4 to 8 hours to spheroidize the complex carbide;

forging the secondarily heat-treated wire rod to form the bearing steel; and

quenching, rapidly cooling, and tempering the formed bearing steel.

In particular, the alloy composition may have a composition comprising, based on a total weight of the composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) being the balance of the weight of the alloy composition. The alloy composition may further comprise one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb). Further, the above alloy compositions may further include an amount of about 0.006 wt % or less of nitrogen (N), an amount of about 0.001 wt % or less of oxygen (O), an amount of about 0.03 wt % or less of phosphorus (P), and an amount of about 0.01 wt % or less of sulfur (S).

The term “carbide complex”, as used herein, refers to a compound comprising at least carbon and other elements that is less electronegative to be positive or partially positive when combined with carbon. The carbide complex may be suitably formed with at least carbon and metal, and the metal may be an alkali metal, an alkali earth metal or a transition metal, a post-transition metal, a lanthanide, or an actinide, without limitation.

The term “spheroidizing” or “spheroidize”, as used herein, refers to a heat-treating process, particularly used for iron based alloy steel or a composition thereof. Particularly, the spheroidizing may refer to a heat-treat process that changes the shape or crystalline shape of carbons of carbide or carbide complex contained in the iron based steel into, for example, globular form, spheroid, or elliptical form, to provide desirable physical properties, such as mechanical strength, high temperature resistance, ductility, machinability and the like. During the spheroidizing, the temperature may be increased upto an iron based alloy steel.

Preferably, the quenching may be performed at a temperature of about 840 to 860° C. for about 0.5 to 2 hours and the tempering may be performed at a temperature of about 150 to 190° C. for about 0.5 to 2 hours.

Preferably, the complex carbide may include one or more selected from the group consisting of M₃C, M₇C₃, M₂₃C₆, and MC carbides, when M is a metal, or particularly, a transition metal.

Preferably, M of the M₃C, M₇C₃, and M₂₃C₆ carbides may comprise one or more selected from the group consisting of chromium (Cr), iron (Fe), and manganese (Mn).

Preferably, the M of the MC carbide be one or more selected from the group consisting of chromium (Cr), iron (Fe), vanadium (V), niobium (Nb), and molybdenum (Mo).

Further provided is a vehicle part that can be manufactured by the bearing steel of the alloy composition as described above.

As such, according to the composition of the present invention, a thickness reduction, a weight reduction, the degree of freedom in design, and the like of vehicles may be facilitated using a bearing steel and costs thereof may be effectively reduced by finely forming a complex carbide and the like in the bearing steel to improve strength, hardness, fatigue life, and the like of the bearing steel and facilitate high strengthening.

DETAILED DESCRIPTION

Terms or words used in the present specification and claims should not be interpreted as being limited to typical or dictionary meanings, but should be interpreted as having meanings and concepts which comply with the technical spirit of the present invention, based on the principle that an inventor can appropriately define the concept of the term to describe his/her own invention in the best manner.

It would be understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

Unless specifically stated or obvious from context, as used herein, the term “about” is understood as within a range of normal tolerance in the art, for example within 2 standard deviations of the mean. “About” can be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values provided herein are modified by the term “about.”

It would be also understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles.

Hereinafter, the present invention will be described in detail. The present invention relates to a bearing steel having improved fatigue durability such as strength, hardness, and fatigue life, which may be applied to engines and transmissions of vehicles and the like, and a method of manufacturing the same.

An alloy composition for bearing steel may comprise: carbon (C), silicon (Si), manganese (Mn), nickel (Ni), chromium (Cr), molybdenum (Mo), aluminum (Al), and copper (Cu), and iron (Fe) being the balance of the weight of the alloy composition. The alloy composition may further comprise one or more selected from the group consisting of vanadium (V) and niobium (Nb). In addition the alloy composition may further comprise one or more selected from the group consisting of nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S).

Preferably, the alloy composition may comprise, based on the total weight of the alloy composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) being the balance of the weight of the alloy composition. Alternatively, the alloy composition may comprise: based on a total weight of the composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb), and iron (Fe) being the balance of the weight of the alloy composition.

In particular, the alloy composition for bearing steel may include a spheroidized complex carbide and the like, and formation thereof may be substantially influenced by an element such as vanadium (V) and niobium (Nb).

The complex carbide may include one or more selected from the group consisting of M₃C, M₇C₃, M₂₃C₆ carbides and MC carbide, when M is a metal or a transition metal, which may be precipitates, and the like. The complex carbide including the aforementioned carbides may improve strength, hardness, and the like of the bearing steel and extend the durability life and the like.

Particularly, M of the M₃C, M₇C₃, and M₂₃C₆ carbides may comprise one or more selected from the group consisting of chromium (Cr), iron (Fe), and manganese (Mn), and M of the MC carbide may comprise one or more selected from the group consisting of chromium (Cr), iron (Fe), vanadium (V), niobium (Nb), and molybdenum (Mo).

Hereinafter, each component of the alloy composition will be described in detail.

(1) Carbon (C)

Carbon (C), as used herein, may improve strength of the steel and stabilize the remaining austenite. The carbon may be included in the alloy composition in an amount of about 0.8 to 1.0 wt %, based on the total weight of the composition. When the content of carbon (C) is less than about 0.8 wt %, strength of the steel may not be sufficiently obtained, and a reduction in fatigue strength and the like may be caused. When the content of carbon (C) is greater than about 1.0 wt %, huge carbide that is not dissolved may remain, thereby reducing fatigue strength, a durability life, and the like and reducing processability before quenching and the like. Accordingly, it is preferable that the content of carbon (C) be in an amount of about to 0.8 to 1.0 wt %.

(2) Silicon (Si)

Silicon (Si), as used herein, may be a deoxidizer and serve to increase strength of the steel by a solid-solution strengthening effect and improve activity of carbon (C). The silicon may be included in the alloy composition in an amount of about 0.35 to 0.9 wt %, based on the total weight of the composition. When the content of silicon (Si) is less than about 0.35 wt %, oxide may not be sufficiently removed and remain in the produced steel thereby deteriorating strength of the steel, and a sufficient solid-solution strengthening effect may not be obtained. When the content of silicon (Si) is greater than 0.9 wt %, decarbonization may occur by an interpenetration reaction in a tissue, such as a site competition reaction with carbon (C) by the excessive content of silicon (Si), and processability may be rapidly reduced due to an increase in hardness before quenching. Accordingly, it is preferable that the content of silicon (Si) be in an amount of about 0.35 to 0.9 wt %.

(3) Manganese (Mn)

Manganese (Mn), as used herein, may improve a quenching property and toughness of the steel thereby improving a rolling fatigue life-span resistance property and the like. The manganese may be included in the alloy composition in an amount of about 0.5 to 1.0 wt %, based on the total weight of the composition. When the content of manganese (Mn) is less than about 0.5 wt %, a sufficient quenching property may not be obtained, and thus processability may be reduced. When the content of manganese (Mn) is greater than about 1.0 wt %, processability before quenching and the fatigue life may be reduced and MnS reducing center segregation may be precipitated. Accordingly, it is preferable that the content of manganese (Mn) be in an amount of about 0.5 to 1.0 wt %.

(4) Nickel (Ni)

Nickel (Ni), as used herein, may serve to micronize crystal grains of the steel, improve solid-solution strengthening, matrix strengthening, low temperature impact toughness, hardenability, and the like, reduce a temperature of an Al transformation point, facilitate expansion of an austenite tissue, and improve activity of carbon and the like. The nickel may be included in the alloy composition in an amount of about 0.6 to 1.5 wt %, based on the total weight of the composition. When the content of nickel (Ni) is less than about 0.6 wt %, an effect of micronization of the crystal grains may not be sufficiently obtained, and a sufficient improvement effect such as solid-solution strengthening and matrix strengthening may not be obtained. When the content of nickel (Ni) is greater than about 1.5 wt %, red shortness and the like may occur in the steel. Accordingly, it is preferable that the content of nickel (Ni) be in an amount of 0.6 to 1.5 wt %.

(5) Chromium (Cr)

Chromium (Cr), as used herein, may improve a quenching property of the steel, provide hardenability, and simultaneously, micronize and spheroidize a tissue of the steel. The chromium may be included in the alloy composition in an amount of about 1.2 to 1.55 wt %, based on the total weight of the composition. When the content of chromium (Cr) is less than about 1.2 wt %, the quenching property and hardenability may be limited, and sufficient micronization and spheroidizing of the tissue may not be obtained. When the content of chromium (Cr) is greater than about 1.55 wt %, an effect to an increase in content may not be sufficient thereby increasing in manufacturing cost. Accordingly, it is preferable that the content of chromium (Cr) be in an amount of about 1.2 to 1.55 wt %.

(6) Molybdenum (Mo)

Molybdenum (Mo), as used herein, may improve the fatigue life of the steel by increasing the quenching property or strength of the steel after tempering. The molybdenum may be included in the alloy composition in an amount of about 0.2 to 0.5 wt %, based on the total weight of the composition. When the content of molybdenum (Mo) is less than about 0.2 wt %, the fatigue life of the steel may be not sufficiently improved, and when the content of molybdenum (Mo) is more than 0.5 wt %, processability and productivity of the steel and the like may be reduced. Accordingly, it is preferable that the content of molybdenum (Mo) be in an amount of about 0.2 to 0.5 wt %.

(7) Aluminum (Al)

Aluminum (Al), as used herein, may serve as a strong deoxidizer and improve cleanliness of the produced steel. Further, the Al maybe reacted with nitrogen (N) in the steel to form nitride and thus micronize the crystal grains. The aluminum may be included in the alloy composition in an amount of about 0.01 to 0.06 wt %, based on the total weight of the composition. When the content of aluminum (Al) is less than about 0.01 wt %, a sufficient effect relating to the deoxidizer, cleanliness, and micronization of the crystal grains may not be obtained. When the content of aluminum (Al) is greater than about 0.06 wt %, a coarse oxide inclusion and the like may be formed to reduce the fatigue life of the steel and the like. Accordingly, it is preferable that the content of aluminum (Al) be in an amount of about to 0.01 to 0.06 wt %.

(8) Copper (Cu)

Copper (Cu), as used herein, may improve hardenability of the steel and the like. The copper may be included in the alloy composition in an amount of about 0.01 to 0.1 wt %, based on the total weight of the composition. When the content of copper (Cu) is less than about 0.01 wt %, an effect of sufficient hardenability improvement may not be obtained, and when the content of copper (Cu) is greater than about 0.1 wt %, since a solid solubility limit may be exceeded, an effect of strength improvement of the steel may be saturated thereby increasing a manufacturing cost and cause red shortness. Accordingly, it is preferable that the content of copper (Cu) be in an amount of about 0.01 to 0.1 wt %.

(9) Vanadium (V)

Vanadium (V), as used herein, may form precipitates such as the carbide and the like, reinforce a matrix tissue through a precipitation reinforcing effect, improve strength and wear resistance, and micronize crystal grains, and enabling high strengthening at the relatively same cooling rate as SUJ2. The vanadium may be included in the alloy composition in an amount of greater than 0 wt % and about 0.38 wt % or less based on the total weight of the composition. When the content of vanadium (V) is greater than about 0.38 wt %, toughness and hardness of the steel may be substantially reduced. Accordingly, it is preferable that the content of vanadium (V) be in an amount of greater than 0 wt % and about 0.38 wt % or less.

(10) Niobium (Nb)

Niobium (Nb), as used herein, may be combined with carbon and nitrogen at high temperatures to facilitate formation of a carbide and a nitride, respectively, and improve strength and low temperature toughness of the steel. The niobium may be included in the alloy composition in an amount of greater than 0 wt % and about 0.02 wt % or less based on the total weight of the composition. When the content of niobium (Nb) is greater than about 0.02 wt %, since an improvement rate of strength and low temperature toughness of the steel may be reduced as compared to the increased content, a manufacturing cost may be substantially increased as compared to an effect that may be obtained. Further, when the content is excessive than the above mentioned range, niobium (Nb) may exist in a solid solution state in ferrite such that impact toughness may be reduced. Accordingly, it is preferable that the content of niobium (Nb) be in an amount of greater than 0 wt % and about 0.02 wt % or less.

(11) Nitrogen (N)

Nitrogen (N) may be an impurity, which may react with aluminum (Al) to form MN and thus reduce the durability life of the steel and the like. Accordingly, it is preferable that the content of nitrogen (N) be limited to about 0.006 wt % or less based on the total weight of the composition.

(12) Oxygen (O)

Oxygen (O) may be an impurity reducing cleanliness of the steel and degrading the steel through contact fatigue. Accordingly, it is preferable that the content of oxygen (O) be limited to about 0.001 wt % or less based on the total weight of the composition.

(13) Phosphorus (P)

Phosphorus (P) may be an impurity inducing segregation of a crystal grain boundary to reduce toughness of the steel. Accordingly, it is preferable that the content of phosphorus (P) be limited to about 0.03 wt % or less based on the total weight of the composition.

(14) Sulfur (S)

Sulfur (S) may increase machinability of the steel to make processing easy, but sulfur (S) may reduce toughness of the steel by grain boundary segregation and be reacted with manganese (Mn) to form MnS and thus reduce the fatigue life of the steel. Accordingly, it is preferable that the content of sulfur (S) be limited to about 0.01 wt % or less based on the total weight of the composition.

As such, the bearing steel comprising the alloy composition may have improved fatigue durability according to various exemplary embodiments of the present invention, and thus, the bearting steel may be applied to vehicle parts and the like. In particular, the bearing steel be applied to bearings of engines and transmissions of the vehicles and the like.

Hereinafter, in another aspect, the present invention relates to a method of manufacturing a bearing steel having improved fatigue durability.

The method of manufacturing the bearing steel having improved fatigue durability may comprise: heat-treating a wire rod comprising an alloy composition, at a temperature of about 720 to 850° C. for about 4 to 8 hours to spheroidize a complex carbide; wire-drawing the heat-treated wire rod; secondarily heat-treating the wire-drawn wire rod at a temperature of about 720 to 850° C. for about 4 to 8 hours to spheroidize the complex carbide; forging the secondarily heat-treated wire rod to form the bearing steel; quenching, rapidly cooling, and tempering the formed bearing steel.

In particular, the alloy composition may comprise, based on the total weight of the alloy composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) being the balance of the weight of the alloy composition. Alternatively, the alloy composition may comprise: based on a total weight of the composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb), and iron (Fe) being the balance of the weight of the alloy composition.

In the method of manufacturing the bearing steel, the complex carbide may be formed and spheroidized in the steel (alloy composition), and the complex carbide may include one or more selected from the group consisting of M C, M₇C₃, and M₂₃C₆ carbides, and MC carbides that are precipitates, when M is a metal or a transition metal. The complex carbide including the aforementioned carbides may improve strength and hardness of the bearing steel and the like and extend the durability life and the like.

Preferably, the M of the M C, M₇C₃, and M₂₃C₆ carbides may comprise one or more selected from the group consisting of chromium (Cr), iron (Fe), and manganese (Mn), and the M of the MC carbide may comprise one or more selected from the group consisting of chromium (Cr), iron (Fe), vanadium (V), niobium (Nb), and molybdenum (Mo).

The quenching may be performed at a temperature of about 840 to 860° C. for about 0.5 to 2 hours, and the tempering may be performed at a temperature of about 150 to 190° C. for about 0.5 to 2 hours.

When the quenching temperature is less than about 840° C. or the quenching time is less than about 0.5 hours, since a quenched tissue may not be formed uniformly, a material deviation may occur. When the quenching temperature is greater than about 860° C. or the quenching time is greater than about 2 hours, the spheroidized complex carbide formed by primary and secondary spheroidizing heat-treating may be dissolved.

Further, when the tempering temperature is less than about 150° C. or the tempering time is less than about 0.5 hours, sufficient physical properties such as toughness of the bearing steel may not be obtained. When the tempering temperature is greater than about 190° C. or the tempering time is greater than about 2 hours, since hardness of the bearing steel and the like may be rapidly reduced, it may be difficult to improve the durability life.

When the primary heat-treating temperature and the secondary heat-treating temperature to spheroidize a complex carbide may be each less than about 720° C. or the spheroidizing heat-treating time is less than about 4 hours, substantial spheroidizing time for the complex carbide may be required, and thus a manufacturing cost may be rapidly increased.

On the other hand, When the primary and secondary heat-treating temperatures are greater than about 850° C., since the formed complex carbide may be dissolved, a possibility of forming a lamella-type complex carbide instead of a spherical complex carbide during a cooling process may be significantly increased.

When the primary and secondary heat-treating times are greater than about 8 hours, a spheroidizing rate of the complex carbide may be slowed to rapidly increase the manufacturing cost.

Example

Hereinafter, the present invention will be described in more detail through the Examples. These Examples are only for illustrating the present invention, and it will be obvious to those skilled in the art that the scope of the present invention is not interpreted to be limited by these Examples.

In order to check physical properties such as hardness and the durability life of the bearing steel manufactured according to the present invention, Comparative Examples 1 to 10 having the components as described in the following Table 1 and Examples 1 to 3 having the components as described in the following Table 2 were manufactured according to the manufacturing method of the present invention.

TABLE 1 Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- Compar- ative Ex- ative Ex- ative Ex- ative Ex- ative Ex- ative Ex- ative Ex- ative Ex- ative Ex- ative Ex- Classification ample 1 ample 2 ample 3 ample 4 ample 5 ample 6 ample 7 ample 8 ample 9 ample 10 C 1.00 0.97 0.95 0.96 0.92 0.91 0.90 0.84 0.83 0.81 Si 0.27 0.32 0.39 0.40 0.92 0.85 0.86 0.25 0.83 0.82 Mn 0.38 0.63 0.62 0.60 0.70 0.72 0.73 0.65 0.68 0.69 P 0.012 0.012 0.010 0.011 0.014 0.013 0.011 0.011 0.012 0.014 S 0.005 0.002 0.005 0.004 0.004 0.005 0.004 0.004 0.003 0.005 Cu 0.05 0.043 0.047 0.044 0.049 0.047 0.048 0.049 0.047 0.048 Ni 0.05 0.62 0.54 0.63 1.43 1.53 1.47 1.49 1.57 1.46 Cr 1.46 1.22 1.2 1.15 1.37 1.38 1.13 1.35 1.33 1.42 Mo 0.02 0.24 0.23 0.25 0.53 0.49 0.46 0.13 0.34 0.35 Al 0.017 0.024 0.023 0.026 0.017 0.018 0.015 0.018 0.015 0.018 N 0.0035 0.0049 0.0042 0.0052 0.0041 0.0042 0.0043 0.0051 0.0044 0.0047 O 0.0006 0.0004 0.0003 0.0004 0.0002 0.0004 0.0005 0.0004 0.0005 0.0003 V — 0.26 0.42 0.78 — 0.44 — 0.16 0.48 0.16 Nb — 0.023 0.019 0.027 0.015 0.013 0.024 — — 0.032 Fe Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Unit: wt %

TABLE 2 Classification Example 1 Example 2 Example 3 C 0.98 0.93 0.82 Si 0.38 0.87 0.81 Mn 0.61 0.71 0.67 P 0.013 0.012 0.01 S 0.004 0.003 0.005 Cu 0.048 0.042 0.041 Ni 0.64 1.47 1.46 Cr 1.23 1.39 1.36 Mo 0.22 0.47 0.36 Al 0.025 0.016 0.017 N 0.005 0.0048 0.0042 O 0.0004 0.0005 0.0003 V 0.2 — 0.16 Nb 0.019 0.014 — Fe Balance Balance Balance Unit: wt %

In Comparative Examples 1 to 10 of Table 1 and Examples 1 to 3 of Table 2, during the manufacturing process, the primary heat-treating temperature was set to about 800° C., the secondary heat-treating temperature was set to about 720° C., the quenching temperature and time were set to about 850° C. and about 1 hour, respectively, and the tempering temperature and time were set to about 150° C. and about 1 hour, respectively.

Herein, Comparative Examples 1 to 10 did not include one or more of vanadium (V) and niobium (Nb), or even though one or more were included, the content range of vanadium (V) or niobium (Nb) exceeded the content range of the present invention, or even though the content range of vanadium (V) or niobium (Nb) satisfied the present invention, the content ranges of one or more of residual components did not satisfy the content range of the present invention.

On the contrary, Examples 1 to 3 included one or more of vanadium (V) and niobium (Nb), the content range thereof satisfied the content range of the present invention, and the content ranges of the residual components also satisfied the content range of the present invention.

As described above, in order to check the difference between physical properties of Comparative Examples 1 to 10 and Examples 1 to 3 having the difference in constitution thereof, the physical properties are compared and arranged in Table 3.

TABLE 3 Rotation number of Hardness rotation bending Durabil- at room fatigue tester at ity life temper- Hardness 150° C./6.2 GPa compar- ature at 300° C. surface pressure ison Classification (HV) (HV) (L10 life, times) (%) Comparative 720 698 8,400,000 100 Example 1 Comparative 752 715 8,550,000 102 Example 2 Comparative 763 721 8,680,000 103 Example 3 Comparative 739 713 8,650,000 103 Example 4 Comparative 775 723 9,200,000 110 Example 5 Comparative 767 724 9,230,000 110 Example 6 Comparative 765 721 9,270,000 110 Example 7 Comparative 758 710 9,330,000 111 Example 8 Comparative 740 708 9,410,000 112 Example 9 Comparative 746 712 9,370,000 112 Example 10 Example 1 842 831 18,791,000 224 Example 2 831 821 17,917,000 213 Example 3 836 828 18,489,000 220

In Table 3, hardenesses at room temperature, hardenesses at 300° C., the rotation numbers of the rotation bending fatigue tester to the L10 life under the surface pressure condition of 6.2 GPa at 150° C., and the durability lives considering these hardenesses and rotation numbers of the Comparative Examples and the Examples are compared.

Herein, in the case of the hardness, the KS B 0811 measurement using the Micro Vickers Hardness tester [Manufacturer: Future Tech, Model: FM-700] was used. As seen through Table 3, it could be seen that the hardness at room temperature of about 25° C. was greater by about 11% in Examples 1 to 3 than in Comparative Examples 1 to 10, and the hardness at 300° C. was also greater by about 16% in Examples 1 to 3 than in Comparative Examples 1 to 10.

The rotation number of the rotation bending fatigue tester was measured at 150° C. and the L10 life of the standard line diameter of 4 mm was measured by the KS B ISO 1143 measurement method where the rotation bending fatigue tester was used. The L10 life is the rating fatigue life of the specimen, and means the total rotation number of the rotation bending fatigue tester until 10% of the specimen is damaged.

In this case, it could be seen that in the case of the rotation number of the rotation bending fatigue tester with respect to the L10 life under the surface pressure condition of 6.2 GPa at 150° C., the average value of Examples 1 to 3 was 18,399,000 times and was about two times greater than 9,009,000 times that was the average value of Comparative Examples 1 to 10.

In order to compare the durability lives of Comparative Examples 1 to 10 and Examples 1 to 3 based on the rotation number of the rotation bending fatigue tester, 8,400,000 times that was the rotation number of the rotation bending fatigue tester of Comparative Example 1 was set as the standard of the durability life of 100%, and based on the rotation number of the rotation bending fatigue tester of Comparative Example 1 as the standard, the increase or the decrease between the rotation numbers of the rotation bending fatigue tester of Comparative Examples 2 to 10 and Examples 1 to 3 was represented as the percentage.

That is, the percentage for comparing the durability lives of Comparative Examples 1 to 10 and Examples 1 to 3 is a value representing the degree of relative increase and decrease of the rotation numbers of the rotation bending fatigue tester of the residual Comparative Examples 2 to 10 and Examples 1 to 3 based on Comparative Example 1.

Herein, through comparison of the durability lives of the Comparative Examples and the Examples, it could be seen that like the rotation number of the rotation bending fatigue tester, the durability life of Examples 1 to 3 was about two times greater than the durability life of Comparative Examples 1 to 10.

As described above, in order to check the reason why hardness and the durability life of the Examples were better than those of the Comparative Examples, the kind and vol % of the complex carbides included in Comparative Example 1 and Examples 1 to 3 are described in the following Table 4.

TABLE 4 Classification Me₃C Me₇C₃ VC + NbC MoC Comparative 10.8 — — 0.02 Example 1 Example 1 10.75 — 0.4  0.54 Example 2 8.97 — — 0.52 Example 3 7.25 — 0.61 0.12 Unit: vol % Me: one or more selected from the group consisting of chromium (Cr), iron (Fe), and manganese (Mn)

In Table 4, the contents of the complex carbides included in Comparative Example 1 and Examples 1 to 3 are compared. As seen in Table 4, the complex carbide of Comparative Example 1 mainly includes Me₃C and a small amount of MoC, but Examples 1 to 3 relatively uniformly include VC and NbC as well as Me₃C and MoC. This difference in constitution of the complex carbides may be considered as one of reasons why the Examples have hardness and the durability life that are better than those of the Comparative Examples.

Therefore, it could be experimentally confirmed that Examples 1 to 3 satisfying the components and the content range according to the present invention and manufactured through the heat-treating process according to the present invention included various complex carbides and the like, and thus had strength and the durability life that were better than those of Comparative Examples 1 to 10.

As described above, the present invention has been described in relation to various exemplary embodiments of the present invention, but the embodiments are only illustration and the present invention is not limited thereto. Embodiments described may be changed or modified by those skilled in the art to which the present invention pertains without departing from the scope of the present invention, and various alterations and modifications are possible within the technical spirit of the present invention and the equivalent scope of the claims which will be described below. 

What is claimed is:
 1. An alloy composition for bearing steel, comprising: an amount of about 0.8 to 1.0 wt % of carbon (C); an amount of about 0.35 to 0.9 wt % of silicon (Si); an amount of about 0.5 to 1.0 wt % of manganese (Mn); an amount of about 0.6 to 1.5 wt % of nickel (Ni); an amount of about 1.2 to 1.55 wt % of chromium (Cr); an amount of about 0.2 to 0.5 wt % of molybdenum (Mo); an amount of about 0.01 to 0.06 wt % of aluminum (Al); an amount of about 0.01 to 0.1 wt % of copper (Cu); and iron (Fe) constituting the balance of the weight of the alloy composition, all the wt % based on the total weight of the alloy composition.
 2. The alloy composition of claim 1 further comprising one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb), all the wt % based on the total weight of the alloy composition.
 3. The alloy composition of claim 1, wherein the alloy composition further comprises an amount of about 0.006 wt % or less of nitrogen (N), an amount of about 0.001 wt % or less of oxygen (O), an amount of about 0.03 wt % or less of phosphorus (P), and an amount of about 0.01 wt % or less of sulfur (S), all the wt % based on the total weight of the alloy composition.
 4. The alloy composition of claim 1, consisting essentially of: an amount of about 0.8 to 1.0 wt % of carbon (C); an amount of about 0.35 to 0.9 wt % of silicon (Si); an amount of about 0.5 to 1.0 wt % of manganese (Mn); an amount of about 0.6 to 1.5 wt % of nickel (Ni); an amount of about 1.2 to 1.55 wt % of chromium (Cr); an amount of about 0.2 to 0.5 wt % of molybdenum (Mo); an amount of about 0.01 to 0.06 wt % of aluminum (Al); an amount of about 0.01 to 0.1 wt % of copper (Cu); and iron (Fe) constituting the balance of the weight of the alloy composition, all the wt % based on the total weight of the alloy composition.
 5. The alloy composition of claim 1, consisting essentially of: an amount of about 0.8 to 1.0 wt % of carbon (C); an amount of about 0.35 to 0.9 wt % of silicon (Si); an amount of about 0.5 to 1.0 wt % of manganese (Mn); an amount of about 0.6 to 1.5 wt % of nickel (Ni); an amount of about 1.2 to 1.55 wt % of chromium (Cr); an amount of about 0.2 to 0.5 wt % of molybdenum (Mo); an amount of about 0.01 to 0.06 wt % of aluminum (Al); an amount of about 0.01 to 0.1 wt % of copper (Cu); one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb), and iron (Fe) constituting the balance of the weight of the alloy composition, all the wt % based on the total weight of the alloy composition.
 6. A method of manufacturing a bearing steel, comprising: heat-treating a wire rod comprising an alloy composition, at a temperature of 720 to 850° C. for 4 to 8 hours to spheroidize a complex carbide; wire-drawing the heat-treated wire rod; secondarily heat-treating the wire-drawn wire rod at a temperature of 720 to 850° C. for 4 to 8 hours to spheroidize the complex carbide; forging the secondary heat-treated wire rod to form the bearing steel; and quenching, rapidly cooling, and tempering the formed bearing steel, wherein the alloy composition comprises: based on a total weight of the alloy composition, an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of about 0.35 to 0.9 wt % of silicon (Si), an amount of about 0.5 to 1.0 wt % of manganese (Mn), an amount of about 0.6 to 1.5 wt % of nickel (Ni), an amount of about 1.2 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), an amount of about 0.01 to 0.06 wt % of aluminum (Al), an amount of about 0.01 to 0.1 wt % of copper (Cu), and iron (Fe) constituting the balance of the weight of the alloy composition.
 7. The method of claim 6, wherein the alloy composition further comprises one or more selected from the group consisting of an amount of more than 0 wt % and about 0.38 wt % or less of vanadium (V) and an amount of more than 0 wt % and about 0.02 wt % or less of niobium (Nb), all the wt % based on the total weight of the alloy composition.
 8. The method of claim 6, wherein the quenching is performed at a temperature of 840 to 860° C. for 0.5 to 2 hours, and the tempering is performed at a temperature of 150 to 190° C. for 0.5 to 2 hours.
 9. The method of claim 6, wherein the complex carbide comprises one or more selected from the group consisting of M₃C, M₇C₃, M₂₃C₆, and MC carbides, wherein M is a metal or a transition metal.
 10. The method of claim 9, wherein the M of the M₃C, M₇C₃, and M₂₃C₆ carbides is one or more selected from the group consisting of chromium (Cr), iron (Fe), and manganese (Mn).
 11. The method of claim 9, wherein the M of the MC carbide is one or more selected from the group consisting of chromium (Cr), iron (Fe), vanadium (V), niobium (Nb), and molybdenum (Mo).
 12. A vehicle part that comprises an alloy composition of claim
 1. 13. The vehicle part of claim 12, wherein the vehicle part is a bearing of an engine or a transmission. 